The present invention relates generally to an internal combustion engine temperature control and, more particularly, to an apparatus and method for controlling temperature of an outboard marine engine using a solenoid assisted blow-off valve.
It is well known that most internal combustion engines use a pressurized cooling system to dissipate heat generated by the combustion process. The cooling system circulates liquid coolant through a coolant jacket which surrounds certain parts of the engine. The heat is transferred from the engine to the coolant in the jacket so as to substantially maintain engine temperature at a predetermined optimum or ideal value. While it is critical not to overheat the engine, it is equally important to maintain higher operating temperatures to minimize exhaust emissions. Further, it is generally understood that engines run more efficiently at fairly high temperatures. To minimize exhaust emissions, optimal control of engine and spark plug temperature is essential. Engine temperature also affects the viscosity of oil used to lubricate the engine. At a lower viscosity, engine parts move more freely, the engine uses less energy, and engine life is therefore extended.
Conventional engine temperature control system thermostats work in conjunction with blow-off valves to regulate engine temperature. The thermostat is configured to open and close based on temperature of coolant circulating through the engine. When the coolant reaches a predetermined temperature, the position of the thermostat changes. For example, when a thermostat is in a closed position, coolant is circulated back to the pump and allowed to be re-circulated through the system. In contrast, when the thermostat is in an open position, the coolant temperature has exceeded the predetermined threshold and therefore coolant is deposited or returned to the coolant source whereupon the pump will then draw additional coolant from the coolant source and circulate the newly acquired coolant through the cooling system. In this regard, coolant having an excessive temperature is replaced by cooler coolant.
To improve fuel efficiency as well as reduce emissions, the thermostat is typically set at a temperature such that the engine is allowed to reach a relatively high ideal operating temperature. However, as engine speed increases, the thermostat can no longer adequately control engine temperature. As such, a blow-off valve or valves are used to allow coolant circulation back to the coolant source when pressure in the cooling system exceeds a predetermined value, regardless of temperature. Blow-off valves typically include a spring that places a biasing force on a head of a valve to maintain the valve in a closed position until cooling system pressure exceeds this biasing force and thereby unseats the head and allows coolant flow through the valve.
Pressure in the cooling system is typically a function of the speed by which the pump circulates coolant through the system. Generally, the pump is driven by the engine and therefore pumps coolant as a function of engine speed. For outboard motors, this arrangement typically results in the blow-off valve opening when the engine speed is approximately 1500 RPM. A drawback of this configuration however is that at mid-range engine speeds, i.e., 1500–2500 RPM, the blow-off valve opens and prevents the engine from running at higher temperatures, thereby negatively affecting emissions. However, it is not sufficient to simply increase the temperature threshold of the thermostat to run the engine at hotter temperatures and increase the pressure threshold of the blow-off valve to increase the cooling system pressure required for blow-off. Such a configuration fails to consider an engine running at lower engine speeds and higher loads. In this range, the increased load on the engine will cause an increase in an engine temperature that cannot be fully accommodated through thermostatic control of coolant circulation. Moreover, because the engine speed is relatively low, the pump is being driven at a speed insufficient to cause a build-up in pressure in the cooling system. As a result, the engine can run too hot and overheat. This linear relationship between thermostat control and blow-off valve control of a conventional system is illustrated in FIG. 1. As indicated, because conventional blow-off valves do not allow engine temperature control as a function of engine load, it is not possible to run the engine hot at mid-range speeds and low loads without over-heating the engine at higher loads for the same speed.
It would therefore be desirable to design a blow-off valve assembly that allows the engine to run at increased temperatures at higher engine speeds that also can be opened when pressure in the cooling system is insufficient to open the valve thereby allowing the engine to run at desirable high temperatures at low speed and low load while preventing overheating when the temperature of the engine cannot be thermostatically controlled.